REAL-TIME COMMUNICATIONS IN A COMPUTER-CONTROLLED WORKCELL

A computer-integrated manufacturing (CIM) system is composed of several workcells, each of which contains robots, NC machines, sensors, and a transport mechanism. Each CIM workcell is controlled by multiple processors interconnected by an intracell bus, but the individual cells are interconnected by an arbitrary network. This paper considers a communication subsystem that is designed to support real-time control and coordination of devices in each CIM cell. The concept of a poll number is proposed to control the access to the intracell bus. The bus access mechanism with the poll number is intended to minimize the probability of real-time messages missing their deadlines. When a CIM task generates a time-constrained message, a poll number is computed for this message according to the message's deadline and the task's prority. When the intracell bus is free, the various tasks at a workcell that desire to use the bus write the poll number onto the bus and read it back, one bit at a time, starting from the most significant bit. If at any time the bit read back is different from the bit written, then the corresponding task drops out of the contention for the bus. Use of a poll number provides not only for decentralized control of the intracell bus, but also a high degree of flexibility in scheduling messages. The performance of the bus access mechanism with a poll number is analyzed and compared with that of a token bus, which is widely used in CIM systems such as MAP networks. The probability of a real-time message missing its deadline in a token bus is found to be much higher than that of the proposed mechanism.